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It is anticipated that unicast addressing
will be used for the vast majority of Internet traffic under IPv6, just
as is the case for older one, IPv4. It is for this reason that the largest
of the assigned
blocks of the IPv6 address space is dedicated
to unicast addressing. A full 1/8th slice of the enormous IPv6 address
pie is assigned to unicast addresses, which are indicated
by a 001 in the first three bits of the address. The question
then is: how do we use the remaining 125 bits in our spacious IP addresses?

Rationale for A Structured Unicast Address Block

When IPv4 was first created, the
Internet was rather small, and the model for allocating address blocks
was based on a central
coordinator: IANA. Everyone who wanted
address blocks would go straight the central authority. As the Internet
grew, this model became impractical. Today, IPv4's classless addressing
scheme allows variable-length network IDs and hierarchical assignment
of address blocks. Big ISPs get large blocks from the central authority
and then subdivide them and allocate them to their customers, and so
on. This is managed by today's Internet providers, but there is nothing
in the address space that helps manage the allocation process. In turn,
each organization has the ability to further subdivide their address
allocation to suit their internal requirements.

The designers of IPv6 had the benefit
of this experience and realized there would be tremendous advantages
to designing the unicast address structure to reflect the overall topology
of the Internet. These include:

Easier allocation of address blocks at various
levels of the Internet topological hierarchy.

IP network addresses that automatically reflect
the hierarchy by which routers move information across the Internet,
allowing routes to be easily aggregated for more efficient routing.

Flexibility for organizations like ISPs to subdivide
their address blocks for customers.

Flexibility for end-user organizations to subdivide
their address blocks to match internal networks, much as subnetting
did in IPv4.

Greater meaning to IP addresses.
Instead of just being a string of 128 bits with no structure, it would
become possible to look at an address and know certain things about
it.

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